1Department of Neurology, University of California School of Medicine, San Francisco, CA 94143, USA.

Abstract

Quantal release of the principal excitatory neurotransmitter glutamate requires a mechanism for its transport into secretory vesicles. Within the brain, the complementary expression of vesicular glutamate transporters (VGLUTs) 1 and 2 accounts for the release of glutamate by all known excitatory neurons. We now report the identification of VGLUT3 and its expression by many cells generally considered to release a classical transmitter with properties very different from glutamate. Remarkably, subpopulations of inhibitory neurons as well as cholinergic interneurons, monoamine neurons, and glia express VGLUT3. The dendritic expression of VGLUT3 by particular neurons also indicates the potential for retrograde synaptic signaling. The distribution and subcellular location of VGLUT3 thus suggest novel modes of signaling by glutamate.

GABAergic, cholinergic, and aminergic neurons express VGLUT3. (A–C) Scattered interneurons in stratum radiatum (Rad) of the hippocampus double stain for both VGLUT3 (A) and GAD (B and C). A subset of GAD-immunoreactive processes within the pyramidal cell layer (Py) also express VGLUT3. However, not all GAD-positive cells express VGLUT3 (arrow). (D–F) In the retina, a subset of cells in the inner nuclear layer (INL) and a discrete layer of synapses in the inner plexiform layer (IPL) label for VGLUT3 (D). Double staining for GAD (E) shows that the VGLUT3-positive cell bodies do not express GAD (F). (G–I) In the striatum, VGLUT3-immunoreactive perikarya and dendrites (G) also express choline acetyltransferase (H and I). (J–L) Processes of a raphe neuron grown in autaptic microculture label for VGLUT3 (J) as well as serotonin (K and L). Although most processes show colocalization, a subset contain serotonin but not VGLUT3 (arrow) whereas others contain VGLUT3 but not serotonin (arrowheads) (L). [Bars = 50 μm (A–C and G–I), 12.5 μm (D–F), and 10 μm (J–L).]